The present invention relates generally to the control of electrically operated apparatus, and particularly to a method and circuit arrangement which allows switching between two regulating modes without disrupting the process being regulated.
In the process of reducing the thickness of metal in a rolling mill, for example, there are times when it is desirable to change from a speed control parameter to that of say a voltage control parameter, and vice versa. For example, in threading a rolling mill, a speed regulating parameter is not a suitable parameter for regulation, as any speed mismatch between the stands of the mill will result in a changing tension on the material between the stands. Hence, another regulating parameter is needed, such as thatprovided by the use of voltage regulating means. Such means provide a "softer" way to control slow rotation of the rolls in the process of threading metal sheet or strip into and through each stand. The process feedback of sheet loading to the drives of the stands forces each drive to "droop" the required amount for a perfect speed match. After the mill is threaded and its speed increased to a run speed for normal processing of the metal, both tension and speed parameters are available for regulating purposes, whereas a voltage regulating method permits control during relatively slow operations of the mill and therefore facilitates the threading process.
However, in switching or changing between a voltage regulation mode and a tension or speed regulation mode, disruption of the rolling process can occur if the magnitudes or levels of the control signals involved are completely different. For example, if the level of a control voltage, as an active, regulating parameter, is substantially below that of the level of an inactive speed reference, a substantial instantaneous increase in speed will be ordered when the mill is switched to a speed regulating mode, thereby placing substantial instantaneous tension on the metal to the extent that the metal may break. On the other hand, if the active, regulating voltage level is high and the inactive speed control level is low, the mill immediately slows, when switched to speed regulation, thereby tending to pile up metal between stands or between payoff apparatus and the first stand.
Therefore, what is needed in the control art is a means to provide a smooth transition between two different regulating modes so as not to upset the process under regulation.